1. Field of the Invention
The invention relates to a load bank for diesel engines and more particularly to a system, apparatus and method that modifies and utilizes a chilled-fluid air conditioning system onboard a marine vessel for creating and/or maintaining an electrical load on one or more diesel engine-powered generators to avoid the deleterious and/or damaging effects of low-load or no-load operation for the diesel engine.
2. Background of the Invention
Marine diesel engine generators are designed for operation at predetermined temperatures and pressures that can only be achieved when the diesel engine powering the generator is operated under load, generally sixty percent of the engine's rated load capacity or greater. The operation of a diesel engine generator at low loads, particularly over a long period of time, can lead to undesirable consequences, among which are incomplete combustion of the diesel fuel resulting in fouled fuel injectors and valves; condensation formation within the engine which can cause the various parts of the internal engine to corrode and can also lead to a breakdown or degradation of the engine's lubricating oil; condensation of exhaust within the engine's exhaust stacks, commonly referred to as “wet stacking,” as well as condensation in the manifolds thereby causing system corrosion and valve damage; system carbon buildup in the exhaust system resulting in the risk of an exhaust system fire; improper seating of the engine's gaskets and seals resulting in oil leaks; and improper seating of the engine's piston rings which will ultimately be responsible for excessive oil consumption and shortened piston and ring longevity thereby leading to reduced horse power for the engine. The foregoing effects of low load operation are cumulative over a period of time.
Load demands on diesel engine generators, particularly those used in marine operations onboard a seafaring vessel, are generally created by the vessel's electrical requirements. Marine engine generators are therefore designed and sized for the maximum anticipated load for providing electrical power to operate the vessel's air conditioning, pumps, motors, galley requirements, and appliances, etc., in the event that all of the vessel's electrical apparatus is on-line at any point in time.
One of the more varying electrical power demands onboard a seafaring vessel, and a common source for low-load engine operation, is created by the vessel's air conditioning system due to the substantial electrical requirements and the fluctuating conditions of the weather. The majority of larger marine vessels, such as yachts, utilize conventional fluid-chilled air conditioning systems to heat and cool the vessel as circumstances warrant. In the cooling mode, these systems employ a circulating heat transfer fluid for removing heat from various compartments and staterooms of the vessel. As shown in FIG. 1, the heat transfer fluid 23, typically fresh water, is pumped through a closed circulation loop 28 that extends through one or more sources of heat transfer, typically one or more chillers or reverse-cycle chillers represented by diagram box 18, for ultimately exchanging its heat with seawater 21 transported through the chiller(s) by the action of seawater pump 19. Once sufficiently cooled, the heat transfer fluid 23 is circulated to one or more air handlers (represented by diagram box 42) distributed throughout various locations of the vessel for absorbing the heat from the air in the vessel's compartments. The heat-absorbed return heat transfer fluid 23 is then circulated back to the chiller(s) by the action of circulating pump 24 where it is cooled once again to complete the air conditioning cycle. The power for operating the chiller(s), pumps and other electrical apparatus in the air conditioning system is derived from diesel engine generator 12 when the vessel is at sea.
The conventional chiller, an example of which is described and illustrated in U.S. Pat. No. 4,926,649, comprises an evaporator in combination with a compressor and condenser for cooling the heat transfer fluid contained within the closed circulation loop. In applications for use onboard marine vessels, electrical power is supplied to the compressor by the diesel engine generator for drawing low pressure refrigerant gas from an evaporator, compressing it, and then discharging it in a higher pressurized gaseous state to a condenser. The condenser in turn condenses the hot gaseous refrigerant into a liquid by transmitting its heat to a second heat transfer fluid, typically seawater, pumped through the condenser. As the sea water is pumped through the chiller condenser, it absorbs the heat from the hot gaseous refrigerant and is returned back to the sea.
In the heating mode, i.e., when it is desired to supply heat to the circulating heat transfer fluid, a reversing valve is employed in the chiller for reversing the flow of refrigerant to the chiller's condenser in order to absorb heat from the sea water and transfer it to the circulating heat transfer fluid. In this mode of operation, the chiller acts as a heat pump and is referred to as a reverse-cycle chiller. A conventional heat pump may also be utilized, particularly when the vessel is relegated to cold climate operations.
As an example, a one hundred foot vessel may employ four 5-ton chillers to satisfy the air conditioning needs of the vessel's compartments. During the summer daytime hours, the heat load for the vessel will be sufficient to require that all of the four chillers be online. The electrical power demand for the operation of the chillers will create a sufficient load on the diesel engine generator(s) thereby more than satisfying the minimum load requirements for the generator(s). After sunset, however, the climate air temperature will drop and the heat load of the vessel will be substantially reduced. As the weather cools, the chillers will begin to stage off one by one, and only one of the four chillers will probably be needed to satisfy the vessel's cooling needs. It is during this time that the diesel engine which powers the generator(s) will be operating under very low-load conditions.
The situation is reversed when the vessel is navigating through a cooler climate or operating in cool-climate conditions. During the evening hours, the heating demand for the vessel will be sufficient to require that all four reverse-cycle chillers be online. Alternatively, resistant in-line water heaters may be employed in lieu of the reverse-cycle chillers. In any evert, their activation will require electrical power for the operation of all the reverse-cycle chillers (or in-line resistant water heaters, as the case may be), and the minimum required load on the diesel engine will be more than satisfied. After sunrise, however, the air temperature will increase and the heating demand for the vessel will be reduced. As the weather temperature increases, the reverse-cycle chillers will stage off one by one, and only one or two of the four chillers will probably be needed to maintain the vessel's heating needs. Once again, the engine generator(s) will be operating under low-load conditions.
3. The Related Art
An example of a refrigeration apparatus powered by a diesel engine generator is described in U.S. Pat. No. 5,584,185, issued to Rumble et al. on Dec. 17, 1996. The refrigeration apparatus comprises a compressor, a water-cooled condenser, a chiller/evaporator and a positive displacement circulating pump, all of which are arranged in heat exchange relationship with a recirculating coolant circuit. The engine and refrigeration apparatus utilize an electronic control system that senses when electrical power is required or when the coolant temperature rises above a datum level so as to initiate a prescribed start sequence for the engine, and further, will automatically shut down the engine when a no-load is sensed for the engine. In the latter circumstance, the engine will remain on standby awaiting a power demand.
Multiple chilled-fluid producers are also disclosed in U.S. Pat. No. 6,240,867 B1, issued to Hoyle et al. on Jun. 5, 2001. The patent discloses their distribution within a watertight zone of a multiple-zoned naval ship for independent operation to avoid or reduce the risk of the vessel's functioning capability when impacted by a missile or torpedo. The chilled fluid producers disclosed may also require a flow of water, either sea or fresh water, into which heat can be rejected. U.S. Pat. No. 4,926,649 issued to Martinez, Jr. on May 22, 1990 also discloses the use of multiple chillers to cool a commercial building in a way that utilizes less energy by turning off one or more of the multiple chillers, and also by varying the total water flow through the chillers.
Various controllers for operating multiple chillers are also disclosed in the patent literature. For example, in U.S. Pat. No. 4,506,516 issued to Lord on Mar. 26, 1985, the use of a microprocessor is disclosed for operating multiple chillers, and in U.S. Pat. No. 4,463,574 issued to Spethmann et al. on Aug. 7, 1984, a controller is disclosed for optimally selecting a combination of chillers having dissimilar efficiency characteristics to efficiently meet a building's air conditioning load. Electric controller systems for efficiently operating air conditioning systems are also known, as for example in U.S. Pat. No. 4,147,296, issued to Spethmann on Apr. 3, 1979, which discloses an electric controller system for reducing and/or limiting a building's electrical power consumption by a proportional amount in order to prevent the power consumption from exceeding a predetermined demand limit; and in U.S. Pat. No. 5,946,926 issued to Hartman on Sep. 7, 1999, wherein a single-circuit, chilled fluid cooling system incorporates a variable flow chilled water distribution system to obtain stable operation at reduced variable flow rates of the circulating chilled fluid.
Finally, various approaches have been taken to compensate for low-load operation of a diesel engine generator onboard marine vessels. For example, load banks have been formulated whereby resistive load elements in the form of heating coils are inserted into a separately fabricated intake line coupled with a seawater pump to receive and discharge seawater from and to the vessel. Heating the seawater in this manner demands electrical power from the generator which in turn creates a load on the diesel engine powering the generator. In addition to requiring added space onboard the vessel, and the associated costs for assembling and incorporating the load bank into the vessel, the coils used to heat the seawater encounter calcification over a period of time due to the seawater's high mineral content. This results in the coils being coated with calcium and other minerals that quickly leads to the inability of the coils to transmit heat to the seawater. Consequently, the calcified coils become an added maintenance item in that they must be descaled by repeated acid washing, or simply replaced. Load banks utilizing this method of operation are available from a variety of sources, one of which is Simplx, Inc. of Springfield, Ill.